An activatable photosensitizer for singlet oxygen is of significance due to its potential application in tumor‐specific photodynamic therapy and diagnosis. In this study, a pH‐activatable system that enables the specific photosensitization of singlet oxygen at a low pH value was developed based on the complexation of a porphyrin‐aniline conjugate with phosphorus(V). As a result of complexation with phosphorus, the water solubility of the porphyrin‐aniline conjugate was improved 100‐fold. Furthermore, this complexation strongly suppressed the singlet‐oxygen‐generating activity of the porphyrin photosensitizer under basic conditions, and the quantum yield of the singlet oxygen photosensitization markedly increased under acidic conditions. Based on these improvements, an extremely high ON/OFF switching ratio of the quantum yield was obtained (>400‐fold).

The Cover Feature illustrates the ON/OFF switching of the photosensitization of singlet oxygen and the fluorescence of a pH‐activatable photosensitizer. The developed system facilitates photosensitization under acidic conditions. Therefore, this system satisfies both the requirements of high photoactivity and low photoinduced side effects for application in the photodynamic therapy of cancer. More information can be found in the Article by H. Horiuchi et al. on page 138 in Issue 3, 2019 (DOI: 10.1002/cptc.201800248).